Fluorinated ion exchange films and membranes are known in the art. The fluorinated ion exchange polymer in such films and membranes can be, for example, one whose functional groups are sulfonic acid groups, or alkali metal or ammonium salts thereof. Typical examples of such polymers are those described in U.S. Pat. Nos. 3,282,875, 3,624,053, and 3,849,243. The fluorinated ion exchange polymer can also be, for example, one whose functional groups are carboxylic acid groups or salts thereof. Typical examples of such polymers are those described in British Pat. No. 1,145,445 and U.S. Pat. No. 3,506,635.
Although such films and membranes have many desirable properties which make them attractive for use in the harsh chemical environment of a chloralkali cell, such as good long-term chemical stability, their current efficiencies are not as high as is desired, especially when the caustic is produced at high concentration. As transport of hydroxyl ion in a chloroalkali cell from the catholyte through the membrane to the anolyte increases, current efficiency drops. Larger amounts of oxygen impurity in the chlorine are thereby produced, and there is a greater buildup of chlorate and hypochlorite contaminants in the brine, which contaminants must be removed and discarded to maintain acceptable cell operation. Current efficiencies of at least 90% are highly desirable.
Accordingly, there is a need for improved films and membranes which will permit cell operation at high current efficiencies, and especially for those which will permit operation at high efficiencies over long periods of time. Additionally, it was desired to find a method for modifying the composition of known membranes so as to obtain membranes which have the high current efficiencies desired.